It's funny, I was just posting about a question I had about chrome plating on dropouts — a little bit different, but similar.

I can see chrome plating on the rear of the drive side dropout, not so much anywhere else besides the drive side chain stay. I think it has to do with the plating process. I've done some plating using Caswell products.. not chrome but zinc/copy cadmium.. but I think it's a similar process. You negatively charge the part (it becomes the cathode) and positively charge the metal that forms the plate (anode), and the plating moves electrically in in the bath in a radiating wave pattern from positive to negative. It forms a sort of cone from each anode positioned around the part, but it can also get pulled around corners to a certain degree. The bit on the rear of the dropout is probably just from less-than-perfect masking.